Effects of the small magnetic field incidence angle on the sheath drop in Wendelstein 7-X (W7-X)

Under typically used magnetic field configurations in W7-X, the flux tubes carrying the highest particle and heat flux in the scrape-off layer (SOL) make grazing contact with the grounded divertor surface (incidence angle ≤ 3^o). The shallow incidence significantly affects the sheath drop (δV_s) and floating potential (V_f) on the divertor surface. In W7-X, it has been observed that at ground potential (V_g), the divertor Langmuir probes (LP) collect significant current (I_g). I_g can have contributions from non-ambipolar drifts, thermoelectric currents and potential difference between non-floating wall (or LP) and plasma. For addressing the third contribution, it is necessary to calculate (δV_s) for shallow incidence because I_g depends on the difference between plasma potential (= Vf + δV_s) and V_g. The conventional methods fail to calculate δV_s for shallow incidence. They neither explain the larger than expected ion saturation current on the LPs nor do they appropriately account for a sheath reversal at near parallel incidence. We propose a simple model based on the idea that for ions, below a certain incidence angle, exiting the quasineutral region is faster along the gyration path than along the parallel to field direction. The calculated I_g based on δV_s from the model, shows a very good qualitative and quantitative agreement with experiments. Same calculation with the conventional sheathdrop give results which are far from qualitative agreement. This shows that our model is successful in capturing the modified ion and electron fluxes at the grazing field incidences. This also implies that the major part of the parallel SOL current in W7-X depends on δV_s on the divertor in attached plasmas. The model is also extended to calculate the current and heat flux received by the divertor and shows good quantitative agreement with the observed heat flux.